libpmcstat: Try /boot/modules if module not found

Modules from ports/pkg are commonly installed to /boot/modules rather than to
the same directory the kernel resides in. Look there if a module is not found
next to the kernel.

Submitted by: mmacy
Reported by: Nick Principe <nap@iXsystems.com>
Approved by: mmacy (mentor)
MFC after: 2 weeks
Sponsored by: iXsystems, Inc.